OF THP] WESTERN ISLES OF SCOTLAND. 381 



silica-percentage and lower specific gravity than the rocks with 

 which they are associated. These cases, though on so much grander 

 a scale, seem comparable to those of the Eskdalemuir and Dunoon 

 dykes and the rocks of lien Hiant. (Seethe Quicksilver Deposits of 

 the Pacific Slope, U.S. Geol. Survey, Monograph xiii. pp. 153-162.)] 



X. SUMMAEY OF EeSULTS. 



The oldest of the Tertiary volcanic rocks of the Western Isles of 

 Scotland (which were provisionally classed as " felstones " in 1874) 

 prove on closer study to belong, for the most part, to the group 

 called by von Eichthofen " propylites.'' This term is used in the 

 present memoir in the sense proposed by llosenbusch, namely, as a 

 " pathological variety *' of the andesites and of their Plutonic 

 representatives. 



The rocks from which these " propylites" of Scotland have been 

 formed find their exact analogues among the andesites of Iceland 

 and the Faroe Islands, which have been so well described by Zirkel, 

 Schirlitz, Osann, Breon, and other authors. But in their present 

 condition the Scottish propylites agree in all essential respects with 

 the altered andesites of Hungary and Transylvania, which have been 

 described by Dolter, Szabo, Koch, and other petrographers, and no 

 less strikingly with the rocks bearing the same name in the Western 

 Territories of North America — the rocks which have been so well 

 illustrated by the researches of Zirkel, Wadsworth, Pecker, Hague, 

 and Iddings. 



These Scottish propylites are distinguished by their dioritic 

 aspect, the alteration which their minerals have undergone, and the 

 development of metallic sulphides in their mass. In this way the 

 original characters of their constituent minerals is often completely 

 lost ; various epidotes and chlorites with much secondary magnetite, 

 biotite, and other minerals being formed at the expense of the 

 original constituents. In their general aspect, in their specific 

 gravity, and in their chemical composition, the propylites of 

 Scotland strikiugl)' agree with those of Europe and North America. 



The propylites are shown to be the oldest of the Tertiary lavas of 

 the district ; as a mass, they underlie the opliitic oli^'ine-basalts of 

 the plateaux, though a, few lava-currents of andesitic type are 

 found intercalcated with the latter. These piopylite rocks form 

 lava-currents, which are generally short and bulky as compared 

 with the basaltic flows ; they also constitute '" cupolas " or " quelle- 

 kuppen,"" and lenticular intrusions (" laccolites "). 



By tracing these much altered rocks to points where the changes 

 produced in them have been less extreme, it can be shown that 

 they represent various types of andesite and of the deep-seated 

 representatives of those lavas, the diorites. Among the amphibolic 

 and mica-rocks, we find hornblende-andesites, hornblende-mica- 

 andesites containing cnstatite, mica-andesiles, and also true diorites 

 and quartz-diorites. Among the chief tyjies of the pyroxenic rocks 

 described are glassy augite-andesites, labradorite-andesites, stony 



